Foamed polymeric products have been made by dispersing or dissolving various materials known as blowing agents into molten polymer. Examples of such blowing agents are air, nitrogen or other gasses, volatile materials which are gaseous at molten polymer temperatures such as hydrocarbons or methylene chloride, and materials which decompose to form gaseous products. The products range from high-void material with polyhedral cells which may be ruptured (open cell foam) to low-void closed-cell material having elongated voids. Siggel et al U.S. Pat. Nos. 4,164,603 and 4,380,594 disclose polymeric filaments having random voids made by injecting dimethylsiloxane nucleating agents (silicone oil) of viscosity 3-400 cp at a rate of 0.1 to 1.0 percent and up to 10 percent of soluble gas or gas forming agent inert to the melt which was then extruded through spinneret capillaries at unusually high jet velocities of 5900 cm/min. or more to form filaments having the desired cavities. Siggel also discloses fluorohydrocarbon as a possible gas forming agent.
Yarns produced from polymers containing blowing agents were described by Scott U.S. Pat. No. 3,095,285 but these yarns were produced from plugged capillaries and had continuous hollow voids that enlarged due to the gas expansion inside the filament during extrusion and quenching. No random voids were disclosed.
Randomelongated voids have also been made in polymeric textile filaments by dispersing polyethylene oxide in the molten polymer, extruding the mixture into filaments and drawing the filaments to give elongated striations of PEO within the polymer. When the filaments are made into fabric and scoured as in Magat & Tanner U.S. Pat. No. 3,329,557 or dyed, a portion of the water-soluble PEO is extracted, leaving at least partial voids. These voids reflect incident light and obstruct transmitted light, giving desirable luster and soil-hiding. However, the degree of PEO extraction depends on the degree of scouring of the fabric, the size of the filament, the molecular weight of the PEO, etc., so that the yarn luster can be quite variable and unpredictable. The cost of the PEO can add considerably to the cost of the product. Furthermore, a filament having, for example, 4% PEO may have 10% lower tenacity than the same filament without PEO.
A fiber having substantially gas-filled closed cells of defined size over a wide range of percent cell content would be greatly desired. Higher strength is usually desired, or at least avoidance of strength loss.